Optical fiber connectors are an essential part of substantially any optical fiber communication system. For instance, such connectors are used to join segments of fiber into longer lengths, to connect fiber to active devices such as radiation sources, detectors and repeaters, and to connect fiber to passive devices, such as switches, multiplexers, and attenuators.
A typical optical fiber connector comprises a housing and a ferrule assembly within the housing. The ferrule assembly comprises a ferrule, which has one or more bore holes to accommodate fibers, and a fiber secured in each bore hole such that the end of the fiber is presented for optical coupling by the ferrule. The housing is designed to engage a “mating structure” having an optical path to which the fiber optically couples during mating. The mating structure may be another connector or an active or passive device as mentioned above. The optical path may be, for example, a fiber in a ferrule, a waveguide in a substrate, a lens, or an optically-transparent mass. The principal function of an optical fiber connector is to hold the fiber end such that the fiber's core is axially aligned with optical pathway of the mating structure. This way, light from the fiber is opitcally coupled to the optical pathway.
It is well known that to effect an optical coupling and minimize Fresnel loss, there must be sufficient “physical contact” between the fiber and the optical path of the mating structure. Generally, adequate physical contact requires that an area of the fiber core contacts the optical path. In common optical applications, this area is at least 62.5 μm, although it should be understood that the area of physical contact will be a function of a system's tolerance to Fresnel loss. For purposes of illustration, however, throughout this disclosure, we will assume a requisite physical contact of 62.5 μm.
There are many factors that affect a connector's ability to make adequate physical contact when mated. Applicants submit that these factors are generally related to (1) the geometry of the end-face of the ferrule, (2) the compressive force of the mated connectors, (3) the ferrule material, and (4) the environmental response. These features are herein referred to as the “PC connector interface parameters” or “PC parameters” for short.
Referring to FIG. 1, the key geometric parameters of fiber undercut, radius of curvature, and apex offset are shown. Fiber undercut is a measure of the fiber's recess within the ferrule and is the distance from the fiber end to the ferrule end face. Radius of curvature is a measure of curve of the ferrule's end face. And apex offset is a measure of the fiber core's offset from the apex of the ferrule end face and is the distance from the centerline of the bore hole in the ferrule to the apex of the ferrule end face. These are well known parameters.
With respect to compressive force parameters, different connectors have different mated forces. The term “mated force” refers to the force applied to the ferrule end face when the connector is mated. This force is typically imparted on the ferrule by virtue of a spring which urges the ferrule away from the connector such that the ferrule end face urges against the mating structure. A standard connector typically has a 2.5 mm diameter ferrule and has a mated force ranging from about 0.5 to about 0.9 kg, while a small form factor (SFF) connector typically has a 1.25 mm diameter ferrule and a mated force ranging from about 0.3 to about 0.5 kg.
With respect to ferrule material, the parameters of interest are Young's modulus and Poisson's ratio. Throughout this application, a zirconia ferrule material is considered in detail, however, it should be understood that the present invention is not limited to this particular material or to the Young's modulus and Poisson's ratio associated with this material.
Environmental response is yet another PC parameter that may affect physical contact. Although many such environmental conditions exist, of particular interest herein is the coefficient of thermal expansion mismatch between the fiber and the ferrule material. Additionally, there is potentially a permanent fiber withdrawal due to the creep of the adhesive used to fasten the fiber to the ferrule.
Traditional approaches for assessing whether a connector is likely to make adequate physical contact involve allowable ranges of the end face radius of curvature, apex offset, and fiber undercut as independent parameters for a given constant force. If the undercut of a particular ferrule is above the maximum allowable undercut, then the ferrule is determined to lack the proper geometric parameters to effect adequate physical contact. Although this approach is a simple and effective way of determining whether adequate physical contact will be made, applicants have found that it is overly exclusive and thereby lowers yields considerably. In particular, this approach ignores the interactions between not only the geometric parameters, but also the other PC parameters described above.
A more recent approach determines allowable undercut as a function of end face radius of curvature with a specific allowable maximum apex offset at a given contact force. (See, GR-326-CORE, Issue 3, (Sept. 1999) Genetic Requirements for Singlemode Optical Connectors and Jumper Assemblies, (herein “GR-326-CORE, Issue 3”), incorporated herein by reference). This method determines what the maximum undercut can be based upon the radius of curvature and a fixed constant value representing the maximum apex offset (i.e., 50 μn). Although this more recent approach acknowledges the interaction of radius of curvature and allowable undercut, and, in so doing, is less restrictive than the prior art approach, applicants believe that it is still overly exclusive and therefore unnecessarily limits yields.
Therefore, a need exists for an approach that determines whether a connector will make adequate physical contact that is not overly exclusive. The present invention fulfills this need among others.